Field of the Invention
The present invention relates to an EL device use front plate which improves light extraction efficiency and relates to a lighting device equipped with the EL device use front plate.
Discussion of the Background
In recent years, in lighting devices such as generally used organic electroluminescent (EL) devices (hereinafter is also referred to as EL device(s) or device(s)), for example, the efficiency of light extraction, i.e. discharging light from a glass substrate into the air, is only 20%. Therefore, considering mass production of panels, for example, films are externally attached to such devices in many cases. Although it depends on the structure of a panel, if a film is externally attached to a device, light extraction efficiency is about 25%, resulting in large waste of light. The reason for the large waste of light is that the refractive index of a substrate, which is mainly made of glass or plastic, is different from that of an organic layer, and light is totally reflected off the interface between the substrate and the organic layer.
To further improve light extraction efficiency, a light extraction structure is also needed to be provided inside the device.
Progress is being made in light extraction efficiency for lighting devices as can be seen in PTLs 1 to 3, for example.
PTL 1: JP-A-H08-083688
PTL 2: JP-A-2009-009861
PTL 3: JP-A-2003-316291